An optical illusion is difference between reality and what the brain thinks its seeing. The information gathered by the eye is processed in the brain to give a image that does not agree with a physical measurement of the source. There are three main types: literal optical illusions that create images that are different from the objects that make them, physiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, tilt, colour, movement), and cognitive illusions where the eye and brain make unconscious inferences.